1. Field of the Invention
This invention relates generally to expanded polystyrene constructs having a high-impact resistance and strength without an unacceptable increase in brittleness, and a method for making the same. More particularly, this invention relates to such constructs that are created by a molding process that includes, as an intermediary step, the imparting of a cam or beveled edge surface to the construct preparatory to compressing the construct to high density. The beveled edge surface imparted to the construct serves to uniformly distribute the forces applied during the compression step even over relatively large angular deviations from the mold parting line, thereby permitting the molding of a single large construct that formerly, prior to this invention, would have required the molding and combining of several smaller, partial constructs. Thus, the disclosed method is particularly useful in the molding of polystyrene motorcyclist and bicyclist safety helmets and polystyrene shipping boxes such as those commonly used for shipping iced fish. This invention also provides molding apparatus adapted for molding constructs of the type described.
2. Background Art
A method for molding an expanded, highly impact-resistant, polystyrene construct from polystyrene beads was disclosed in U.S. Pat. No. 5,718,968 to P. W. Cutler et al., which by this reference is incorporated herein. The polystyrene beads include a thin outer shell of polystyrene and a hollow interior that includes a blowing agent, such as pentane, for example. According to the method, the beads were first pre-expanded by application of heat through hot air or steam, which caused the blowing agent to vaporize and expand the beads. The beads were then cooled, which caused the beads to have reduced internal pressure. The pre-expanded beads were next placed into a mold assembly where they were subjected to heat and pressure for molding to a first volume. The molded article was then rapidly cooled in the mold assembly, thereby causing the vaporized blowing agent within the beads to condense and create a pressure less than atmospheric inside the beads. Thereafter, the molded product with beads having low internal pressure was immediately subjected to compression within the mold assembly to a second volume, less than the first volume. This resulted in a single layer construct that had memory shape and was particularly suited for acoustic and thermal insulation. Alternatively, additional layers were molded within the mold assembly by adding beads to the first molding step and then molding together the originally-molded layer with the additional beads. This yielded a layered construct wherein each layer had a different density than the other layers and each layer had a density higher than the density of the beads from which it was molded, except when a last layer was a backfill layer of density equal to that of the beads from which it was molded. The layered construct was volume stable, high density, high strength, and highly impact resistant.
When this method was used to mold a safety helmet for a motorcyclist or bicyclist, the outermost layer had the highest density, while inner layers had lower densities to absorb impact forces to minimize transmission of these forces through the construct. Such a helmet, being substantially hemispherical and having a 360 degree circumference, was thought to require molding in at least five parts--crown, front, back, and two sides--which thereafter were assembled together in a single mold and finally molded together. This was because it had been found that the compression process was less effective when the pressure is applied at angles between 46 degrees and 89 degrees to the parting line of the mold. It was theorized that compression pressure was diffused so that the compression ratio rapidly diminished with pressure applied at angles more than 45 degrees with respect to the parting line of the mold.
There remains a need, therefore, for a method and mold assembly for molding pre-expanded polystyrene beads that permits molding a substantially hemispherical polystyrene construct as a whole, in a single, integrated molding process, thereby eliminating the need for first molding two or more individual component parts thereof and the combining of the parts through an additional molding step. There also remains a need for expanded polystyrene constructs that are produced by the method and mold assembly described herein, which feature high impact resistance and strength without an unacceptable increase in brittleness, and which can be molded in substantially hemispherical shapes in a single, integrated molding process. These needs are fulfilled by the present invention.